NTC thermistors known in the art have been roughly categorized into two types depending on the usage, and temperature-compensating thermistors and inrush current-limiting thermistor. Among these, inrush current-limiting NTC thermistors are mainly built into power circuits and used for limiting the large inrush current that instantaneously flows when the capacitors in the circuits start charge accumulation upon turning on the power source.
One example of the above-described NTC thermistors known in the art is a multilayer NTC thermistor shown in FIG. 3. In this multilayer NTC thermistor, for example, internal electrode layers 11 are embedded in a ceramic element body 20 having a negative resistance temperature characteristic and extend to be exposed in two end faces in an alternating manner. External electrodes 12 are formed on the two end faces of the ceramic element body 20 and are electrically connected to the exposed internal electrode layers 11.
Various thermistor ceramic compositions that contain metal oxides containing manganese (Mn) and nickel (Ni) as main components have been known as the material for the ceramic element body.
For example, Japanese Unexamined Patent Application Publication No. 62-11202 (Patent Document 1) describes a thermistor composition including an oxide containing three elements, namely, manganese, nickel, and aluminum, in which the ratios of these elements are within the ranges of 20 to 85 mol % manganese, 5 to 70 mol % nickel, and 0.1 to 9 mol % aluminum, the total of the three elements being 100 mol %.
Another example, Japanese Patent No. 3430023 (Patent Document 2), describes a thermistor composition in which 0.01 to 20 wt % cobalt oxide, 5 to 20 wt % copper oxide, 0.01 to 20 wt % iron oxide, and 0.01 to 5.0 wt % zirconium oxide are added to a metal oxide, containing, in terms of the content of the metals only, 50 to 90 mol % manganese and 10 to 50 mol % nickel totaling to 100 mol %.
Another example is Japanese Unexamined Patent Application Publication No. 2005-150289 (Patent Document 3) which describes a thermistor composition containing a manganese oxide, a nickel oxide, an iron oxide, and a zirconium oxide, in which a mol % (wherein a is 45 to 95 excluding 45 and 95) manganese oxide in term of Mn and (100-a) mol % nickel oxide in terms of Ni are contained as main components, and per 100 wt % of these main components, the ratios of the respective components are 0 to 55 wt % (excluding 0 wt % and 55 wt %) iron oxide in terms of Fe2O3 and 0 to 15 wt % (excluding 0 wt % and 15 wt %) zirconium oxide in terms of ZrO2.
Meanwhile, COUDERC J. J., BRIEU M., FRITSCH S, and ROUSSET A., DOMAIN MICROSTRUCTURE IN HAUSMANNITE Mn3O4 AND IN NICKEL MANGANITE, THIRD EURO-CERAMICS, VOL. 1 (1993) pp. 763-768 (Non-Patent Document 1) reports a thermistor ceramic composition in which plate-shaped deposits which are generated by gradually cooling Mn3O4 from high temperature (cooling rate: 6° C./hr) but not when Mn3O4 is rapidly cooled from high temperature in air, giving instead a lamella structure (stripe-shaped contrast structure). In addition, this document also reports that NiO0.75Mn2.25O4 forms a spinel single phase when gradually cooled from high temperature (cooling rate: 6° C./hr) in which no plate-shaped deposits or lamella structures are observed, and forms a lamella structure but not plate-shaped deposits when rapidly cooled from high temperature in air.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 62-11202    Patent Document 2: Japanese Patent No. 3430023    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-150289    Non-Patent Document 1: COUDERC J. J., BRIEU M., FRITSCH S, and ROUSSET A., DOMAIN MICROSTRUCTURE IN HAUSMANNITE Mn3O4 AND IN NICKEL MANGANITE, THIRD EURO-CERAMICS, VOL. 1 (1993) pp. 763-768